The present invention is directed to an electric motor protector utilized to break electrical connection to the motor in the event of overheating, overload or other malfunction. More particularly, the invention is directed to a slow make/slow break motor protector which has been improved to facilitate protection of the motor and to provide safety in the use thereof. Motor protectors have long been utilized in conjunction with electric motors to provide a method to eliminate electrical power to the motor in the event that the motor malfunctions. Typically, an electrical motor will include stator coils comprising both a starting winding and main windings. The motor protector is electrically coupled with these windings so as to break the circuit supplying electrical energy to the windings upon the occurrence of a malfunction such as a locked rotor, motor overload, or other similar malfunctions which may produce overheating of the motor. Such overheating may subsequently lead to the motor presenting a substantial safety hazard if a suitable motor protector is absent from the circuit.
The motor protector is usually connected in series to the windings of the stator coil which may include both the start and main windings. In contrast to a fuse-type device utilized as a circuit breaker, motor protectors will effectively break the circuit upon malfunction of the motor, after which the motor will be given a time period in which to cool. Upon coolings the motor protector will act to electrically reconnect the circuit to enable operation of the motor. If the malfunction causing actuation of the motor protector has not been corrected, another similar cycle of breaking the circuit, cooling and reconnecting the circuit will be performed. This process will continue until the motor malfunction is corrected.
There are known various motor protector devices in the prior art which function substantially as described above, and which have been in continuous use over the past few decades without any substantial change in their design. Several known devices include what may be termed a "snap action" type of motor protector, such as that produced by Texas Instruments in their motor protector model 2AM. In these types of devices, electrical connection is made between contacts formed in association with the motor protector wherein one contact is movable relative to the other so as to make or break the electrical circuit. The structure on which the movable contact is positioned is made to go through a center position in opposition to high mechanical stress, after which the component and contact associated therewith will snap to make or break the electrical circuit through the motor protector. These devices normally have a wide differential between open and closed contact modes wherein the intermediate movement of a movable contact is substantial to enable the snapping phase to occur so as to make or break the circuit.
Another device known in the art may be termed "slow" make/slow break device wherein a cantilever arm is made to move slowly in response to temperature differentials encountered in the system. The cantilever arm carries a contact relative to a fixed contact associated with the motor protector, wherein movement of the cantilever arm will cause making or breaking of the electrical connection between contacts. In this type of construction, the cantilever arm moves slowly and has a very small movement differential between open and closed positions. The cantilever arm is constructed such that it will move in response to temperature differentials encountered. General overheating of the motor will cause operation of the motor protector to break the electrical circuit in a gradual manner. Additionally, the motor protector may include a shunt offset portion which is heated upon a malfunction such as a locked rotor condition in the motor so as to radiate heat to the cantilever arm to effect movement thereof so as to open the contacts and break the electrical circuit quickly. After breaking the electrical circuit, the motor along with the motor protector will cool such that the cantilever arm will also cool and will slowly move to its initial position wherein the contacts will be closed and the electrical circuit will be made.
In the above devices, several deficiencies have been found which have reduced their effective use with various electrical motors and the particular applications in which they are used. Although motor design has changed significantly in the recent past, the design of the motor protectors utilized therein have remained substantially constant in the recent past. For example, the design of motors has been drastically modified in response to escalating costs and higher efficiency requirements. Materials have been taken out and material substitutions made to maintain relatively low costs, the result of which tend to make the motors run hotter and limit the useful life thereof. Similarly, the electrical efficiency of the motor has been of increasing importance and actual mandates imposed by industry regulation have resulted in substantial internal design changes in these motors. The ultimate effect of these motor design changes have resulted in motors which will run hotter and at higher speeds, and these new motors are significantly more burdensome of the motor protectors than previously encountered. The effect of the motor design changes referred to above on the motor protectors which have been relied upon in the prior art is to reduce the useful life of the motor protectors as well as to make it harder to meet the regulations and requirements regarding such motor protectors as set by the Underwriters Laboratory (UL). As for all motor protectors, the UL requirements provide for an 18 day testing period wherein the motor is placed in a locked rotor condition and the performance of the motor protector is observed and analyzed over the 18 day period. The 18 day test is required of all motor protectors and must be verified for each motor with which the protector is to be used. The stringent requirements of the 18 day test results in the necessity to provide motor protectors which are rugged and durable in their operation, and which maintain predetermined operating characteristics for the entire 18 day period. As an example, all electric motor protector UL requirements involve 18 day, 24 hour per day continuous locked rotor operation as a minimum, and after the 18 day period the motor must be capable of running a normal operation with the rotor unlocked at the conclusion of the 18 day test. UL also requires the motor manufacturers to submit two motors when a temperature tolerance of .+-.7.degree. C. is relied upon. As every model of motor on which a motor protector is to be used, must be tested under the UL requirements, submission of two only one motor need to be submitted when the temperature tolerance of .+-. 5.degree. C. is used. It is therefore also a design consideration of the motor protector to maintain the lower temperature tolerance by calibration methods so as to reduce the cost of testing under the UL requirements. As an example, the UL 18 day motor locked rotor test for a class motor is conducted in the following manner. With the locked rotor condition, the operation of the motor will quickly result in high operating temperatures which are designed to be prevented by the motor protector. Thus, with a locked rotor condition, the motor protector will be cycled through its operation repeatedly and will be relied upon to break the electrical circuit supplying operating power to the motor. Upon breaking of the circuit, the motor and motor protector will gradually cool after which the motor protector will act to recouple electrical power to the motor, this cycle being repeated over the entire 18 day test. During the first hour of the UL stator testing, the stator peak temperature must not exceed a maximum of 225.degree. C. Additionally, during the first three days of the test, the peak temperature must not exceed 200.degree. C. and the average temperature must not exceed 175.degree. C. Normally, the UL testing procedure monitors the temperature using a type J iron and constantan thermocouple located on the motor windings. Conventionally, the 12 o'clock thermocouple position on the stator windings is utilized as the point where temperatures are measured as it is normally the hottest location on the motor.
It should be evident that the UL requirements including the 18 day locked rotor test place design requirements on the motor protectors wherein the cycle time between breaking of the electrical circuit by the motor protector and subsequently recoupling the circuit should be long enough to allow the motor to cool substantially before operation begins again. In this way, the average temperatures are maintained at a point well below the UL maximums. Additionally, the on-time wherein the electrical circuit is completed cannot be so long as to allow the peak temperatures to exceed the UL maximums. Thus, ideally the motor protectors should provide durable and repeated performance wherein the on-time of the circuit in a locked rotor condition is maintained very short to keep peak temperatures down, and the cycle time is relatively long, to allow sufficient cooling of the motor to maintain average temperatures below the UL limits. In the known devices, the snap action type motor protectors have a cycle time of 2 to 21/2 minutes which has been found to be a very desirable cycle time to allow sufficient cooling (but also to not allow cooling to such a degree that restarting of the motor will have adverse effects thereon). Although the snap action motor protectors have a relatively lengthy cycle time, other deficiencies are possible in their operation. For example, the construction of the snap action type motor protector is such that if the device fails, the wide differential between open and closed positions of the movable contact is lost early in the cycle life of the device. After such differential is lost, the device begins to act like a "creeper" or slow make/slow break device wherein the small contacts provided thereon are not designed for such rugged fast cycling performance and so leading to complete failure of the device. Additionally, as the movable contact goes through the center position which is an area of high mechanical stress, it is possible for the device to fail in a closed position. In this situation, the electrical circuit will be made to allow operation of the motor without the motor protector operating so as to create a very dangerous condition as the motor continues unabated to overheat.
Alternately, in the prior art slow make/slow break devices, it has not been possible to provide a long time duration cycle time which corresponds to the snap action type devices. As an example, one known slow make/slow break device manufactured by assignee of the present invention in their Model 325, shows a device which has a cycle time of approximately 50 seconds. This cycle rate is relatively short, compared to some snap action type devices which renders them disadvantageous for some motor applications. The advantage of the slow make/slow break device is found in that if the device fails, it will fail in an open circuit breaking condition so as to render the motor inoperative and avoid any potential dangerous conditions thereby.